The present invention relates to an impedance matching circuit for a high frequency amplifying circuit and, more particularly, to an impedance matching circuit capable of preventing a phase characteristic from being deteriorated by a change in input level.
In a conventional high frequency amplifying circuit with an impedance matching circuit, an increase in input level causes an output level to be saturated. As a result, the phase characteristic of a signal available with the amplifying circuit is noticeably deteriorated, compared to a condition wherein the input level is low. When the amplifying circuit deals with a QPSK (Quadrature Phase Shift Keying) or similar modulated signal, its phase characteristic has influence on the channel quality. Hence, the circuit is required to cause a minimum of deterioration in the phase characteristic of a signal. Moreover, when the phase characteristic of a signal undergoes an extreme change, the circuit is apt to perform an operation other than the expected linear operation, e.g., parametric oscillation.